CFBDSIR J1458+1013B: A Very Cold (>T10) Brown Dwarf in a Binary System

Abstract

We have identified CFBDS J1458+10 as a 0.11" (2.6 AU) physical binary using
Keck laser guide star adaptive optics imaging and have measured a distance of
23.1+/-2.4 pc to the system based on near-IR parallax data from CFHT. The
integrated-light near-IR spectrum indicates a spectral type of T9.5, and model
atmospheres suggest a slightly higher temperature and surface gravity than the
T10 dwarf UGPS J0722-05. Thus, CFBDS J1458+10AB is the coolest brown dwarf
binary found to date. Its secondary component has an absolute H-band magnitude
that is 1.9+/-0.3 mag fainter than UGPS J0722-05, giving an inferred spectral
type of >T10. The secondary's bolometric luminosity of ~2 x 10^{-7} L_sun makes
it the least luminous known brown dwarf by a factor of 4-5. By comparing to
evolutionary models and T9-T10 objects, we estimate a temperature of 370+/-40 K
and a mass of 6-15 Mjup for CFBDS J1458+10B. At such extremes, atmospheric
models predict the onset of novel photospheric processes, namely the appearance
of water clouds and the removal of strong alkali lines, but their impact on the
emergent spectrum is highly uncertain. Our photometry shows that strong CH4
absorption persists at H-band; the J-K color is bluer than the latest known T
dwarfs but not as blue as predicted by current models; and the J-H color
delineates a possible inflection in the blueward trend for the latest T dwarfs.
Given its low luminosity, atypical colors and cold temperature, CFBDS J1458+10B
is a promising candidate for the hypothesized Y spectral class. However,
regardless of its ultimate classification, CFBDS J1458+10AB provides a new
benchmark for measuring the properties of brown dwarfs and gas-giant planets,
testing substellar models, and constraining the low-mass limit for star
formation.Comment: Tiny cosmetic changes made to match final published version,
including update of Ross 458C values in Table 6 and correction to Figure 7
legen